1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to golf balls having a core and a cover, with dimples being formed on the cover.
2. Description of the Related Art
Generally speaking, golf balls other than those manufactured for use in practice ranges have a core and a cover. There exist cores composed of a single solid rubber layer, two or more solid rubber layers, a synthetic resin layer in addition to a solid rubber layer, and the like.
Numerous dimples are formed on the surface of the cover. The role of the dimples involves causing turbulent flow detachment through promoting turbulent flow transition of a boundary layer by disrupting the air flow around the golf ball during its flight. By promoting turbulent flow transition, the detachment point of the air from the golf ball shifts backwards leading to the reduction of a drag coefficient (Cd) so that the flight distance of the golf ball is prolonged. In addition, the difference of detachment points on the upper and lower sides of the golf ball resulting from back spin is increased by the promotion of turbulent flow transition. Therefore, the, lift force that acts on the golf ball is elevated.
Various golf balls that are provided with an improved dimple pattern to which the improvement of the flight performance is intended have been proposed. For example, JP-A-50744/1983 discloses a golf ball having dimples densely arranged so that pitches between the dimples are equal to or less than 1.62 mm as much as possible. Further, JP-A-192181/1987 discloses a golf ball having dimples densely arranged so that any new dimple can not be formed having an area greater than the average area, on the land parts that are parts other than the original dimples. Moreover, JP-A-347177/1992 discloses a golf ball having dimples, significantly densely arranged, so that the number of the land parts on which a rectangle having a predetermined size can be depicted is equal to or less than 40.
Any of the golf balls disclosed in these known literatures has dimples that are densely arranged. In other words, they have an increased surface area occupation ratio of the dimples. One skilled in the art acknowledges that a critical factor that influences the effects of dimples is the surface area occupation ratio.
Performances of a golf ball on which great importance is placed include spin performances. High back spin speed results in small run (a distance from the position where the golf ball dropped to a position where it stopped, also referred to as “roll”). In other words, for golf players, golf balls which can be spun backwards tend to stop at a targeted position. High side spin speed results in the possibility of curving the golf ball. In other words, for golf players, golf balls which can be side spun tend to intentionally curve. Thus, golf balls that are excellent in spin performances are excellent in control performances. Senior golf players particularly place great importance on control performances upon impact with a short iron.
In general, an ionomer resin is used for the golf ball cover. Ionomer reins are excellent in durability and resilience. However, ionomer resins, in general, have high hardness. Golf balls having a cover composed of an ionomer resin are also inferior in control performances. A cover in which a hard ionomer resin and a ternary copolymer-based soft ionomer resin are used in combination has been proposed with the intention of improving the control performances. Furthermore, a cover in which an ionomer resin and a thermoplastic elastomer are used in combination has been also proposed. In addition, a cover composed of a thermosetting polyurethane elastomer or a thermoplastic polyurethane elastomer has been also proposed.
Travel distance and control performances, which are required performances for a golf ball, are conflicting performances. Accordingly, a golf ball that is satisfactory enough to golf players has not yet been obtained. Thus, an object of the present invention is to provide golf balls which are excellent in both flight performance and a controlled performance.